1. Field of the Invention
The present invention relates to audio signal processing.
2. Description of the Related Art
An audio recording function is implemented in various kinds of electronic devices such as digital video cameras, digital still cameras, cellular phone terminals, and personal computers. When audio recording is performed using such an electronic device having an audio recording function in an environment in which wind is blowing, such an arrangement has a problem of noise in the recorded audio data, which is referred to as “wind noise”. In order to solve such a problem, an arrangement is known in which a wind shield is provided to a microphone, which reduces such wind noise to a certain extent. As another approach, wind noise reducing techniques using signal processing have been proposed (Patent document 1).
The wind frequency spectrum is concentrated in a range that is equal to or lower than 1 kHz. Thus, with conventional techniques, detection of whether or not wind noise occurs is made based on the frequency spectrum acquired by means of a microphone. When wind noise is detected, the L-channel audio signal and the R-channel audio signal are passed through a high-pass filter, so as to reduce the wind noise spectrum component that is equal to or lower than the cutoff frequency of the high-pass filter. A related art has been disclosed in Japanese Patent Application Laid Open No. H10-126878.
The present inventors have investigated such a technique in which the cutoff frequency of such a high-pass filter is controlled according to the magnitude of the wind noise, and have come to recognize the following problems.
In a range in which the magnitude of the wind noise is smaller than a predetermined minimum value, the cutoff frequency of the high-pass filter is set to a predetermined minimum value so as to substantially disable the high-pass filter. With such an arrangement, when the magnitude of the wind noise becomes greater than the predetermined minimum value, the cutoff frequency of the high-pass filter is raised at a predetermined rate.
Such a control operation has a problem of discontinuous change in the rate at which the cutoff frequency is changed at the time point at which the cutoff frequency fc becomes grater than the predetermined minimum value fMIN. Thus, when the magnitude of the wind noise changes such that it straddles the predetermined minimum value, this leads to a sudden change in the cutoff frequency fc, which causes the user to experience auditory discomfort.